The invention concerns a hydraulic steering arrangement, having a steering valve, which connects a steering motor with a pump, and which has a slide with two front sides, the slide being adjustable by means of a pressure fluid both through a hydraulic steering unit with a steering hand wheel and through an electro hydraulic steering unit with a control pressure generator, the two steering units having working connections and the front sides of the slide being connected with shuttle valves, each of which being connected with the hydraulic steering unit on the one side and with the electro hydraulic steering unit on the other side.
A steering arrangement of this kind is known, for example, from the post-published German patent application 199 31 143.9. In this document, each front side of the slide is connected direct in series with a shuttle valve. Each of the two shuttle valves is acted upon by a working pressure of the hydraulic steering unit and a working pressure of the electro hydraulic steering unit. In this connection, each front side of the slide is acted upon by the higher working pressure available at its series-connected shuttle valve. Thus, the steering valve is operable both via the hydraulic and the electro hydraulic steering unit. As the hydraulic steering unit can build up the largest working pressure of the steering arrangement, the hydraulic steering unit can at any time be used to control the vehicle. In this connection, the reaction time of the steering unit depends on the pressures, which act on both front sides of the slide.
The invention is based on the task of reducing the reaction time.
In a hydraulic steering arrangement as mentioned in the introduction, this task is solved in that a valve arrangement is provided, which, during normal operation, connects the working connection of both steering units, which has the highest pressure, with one front side of the slide, and the working connection of both steering units, which has the lowest pressure, with the other front side of the slide.
Thus, it is ensured that during normal operation of the hydraulic steering unit the highest available pressure always works together with the lowest pressure at the steering valve, meaning that the largest pressure difference acts across the slide. During normal operation, the lowest pressure comes from a tank connection of the hydraulic steering unit, assuming that both working connections R, L of the hydraulic steering unit have the same pressure, when being connected at the same time with the tank connection T. During normal operation, this is the case when only the electro hydraulic steering unit is activated. In this connection, the tank connection is connected with a tank or another pressure sink. During normal operation this gives the maximum possible pressure difference and the shortest possible reaction time.
Preferably, the valve arrangement has two branches, and in each branch one working connection of the hydraulic steering unit or one working connection of the electro hydraulic steering unit is connectable with one front side of the slide. Each of the two branches thus combines two paths, which are acted upon by pressure from the various steering units. This simple arrangement of the pressure paths reduces the costs. Additionally, the arrangement of the two pressure paths of a steering unit in different branches provides an improved working reliability of the steering arrangement.
Further, it is preferred that each branch has one path for a higher and one path for a lower of two working pressures. Thus, a predetermined path is allocated to the individual pressures according to their size and relative to the other pressure of the same path, which enables simplified switching arrangements and thus also a more reliable switching operation.
It is particularly preferred that each branch has at least one cross connection to the other branch, through which an adjustment pressure can act upon a valve of the other branch. This increases the number of switching functions of the valve arrangement, which provides an optimised pressure admission of the steering valve.
It is advantageous that the hydraulic steering unit supplies a higher pressure than the electro hydraulic steering unit. This ensures that the vehicle can at any time be steered by means of a manual operation of the hydraulic steering unit. This ensures a high reliability of the steering arrangement.
Further, it is advantageous that each of the two branches has at least two three/two-way valves and at least one shuttle valve. These valves provide a large number of switching opportunities, the simple embodiment of the individual valves reducing the costs and ensuring a stable operation.
Preferably, the first three/two-way valve of a branch, seen from a steering unit, is controlled so that it is penetrable by the lower of the two pressures ruling at the working connections of the steering unit. Thus, the lower pressure of the two branches is already branched at an early stage. This ensures that it is with a high reliability available in both branches, to act upon the front sides of the slide, each being connected in series with one of the two branches.
Additionally, it is preferred that the first three/two-way valve of each branch has two inlet connections, each connected with a steering unit, and an outlet, which is connected with an inlet connection of a downstream-connected second three/two-way valve, the first three/two-way valve being operable via adjustment connections, which again are connected with one of the connections between the inlet connections and the steering units. This simple embodiment of the valve enables an inexpensive and reliable switching of two ruling pressures in such a way that the lower pressure is passed on.
It is advantageous that the shuttle valve of a branch has two working connections, one being connected with the hydraulic steering unit, one with the electro hydraulic steering unit. The shuttle valve has a very simple embodiment and is penetrable by the higher of two pressures. Thus, the shuttle valve arranged here ensures lower costs and a reliable passing on of the higher of the ruling pressures.
It is advantageous that the outlet of the shuttle valve is connected with an inlet connection and an adjustment connection of the second three/two-way valve and with the cross connection. The consequence of this is that the highest pressure ruling in any of the two branches is now available as adjustment pressure in both branches, thus permitting an optimisation of the switching processes.
Further, it is advantageous that the cross connection connects the outlet of the shuttle valve of the one branch with an adjustment connection of the second three/two-way valve of the other branch. This permits that in an operation reliable and cost effective way the highest pressure ruling in the other branch is always acting as adjustment pressure on the two second three/two-way valves.
It is favourable that the second three/two-way valve of a branch, seen from the direction of a steering unit, is adjustable via an adjustment connection, which is connected with one of the cross connections, and via an adjustment connection, which is connected with the outlet of the shuttle valve of the branch, and has an outlet, which is connected with a front side of the slide. This arrangement makes it possible that the connection of a branch with the downstream-connected front side of the slide is operable by means of pressures from both branches. Thus, the pressure application of the valve can be optimised.
Further, it is advantageous that the highest of the pressures, which are passed on by the two shuttle valves of the valve arrangement, operates the second three/two-way valves of the two branches. Thus, the two connections between the branches and their downstream-connected front side of the slide are operated by the highest available pressure. This causes a simplified and reliable adjustment of both switching processes.
It is advantageous that each of the second three/two-way valves is operated so that the one of the second three/two-way valves of both branches being acted upon by the highest of the pressures passed on by the two shuttle valves of the valve arrangement, passes on this pressure to the one front side of the slide, and the other second three/two-way valve passes on the lower of the pressures, which rules at the two inlet connections, to the other front side of the slide. Thus, it is ensured that between the two front sides of the slide, which are acted upon by one of the pressures of the series-connected branch, the highest possible pressure difference is ruling. For the normal operation, this means that on one side the steering valve is connected with the working pressure of a tank connection of the hydraulic steering unit and on the other side with the working pressure of the pump connection of the activated steering unit. In this way the shortest possible reaction time for the steering is obtained.